Such knives used for cutting open the rubber or adhesive beading around automobile windscreens generally comprise a flat or possibly slightly convex cutting face and in operation are mounted on an oscillating head which performs very rapidly oscillating rotary movements over a small angular range. For this purpose the knife is mounted on a spindle which serves as a tool carrier and which is in turn mounted on the head of a tool or driving assembly in such a way that it is rotatable through the aforementioned small angular range.
The prior art knives are either Z- or U-shaped when viewed from the side, one leg of the U or the Z in each case constituting the cutting face or cutting edge, while the knife if fixed to the tool carrier or spindle by the other leg. The intermediate member or adaptor which connects the two legs and which extends substantially at a right-angle to them serves above all to permit the knife also to be used in inaccessible places such as, for example, in the case of motor car windscreens, between the supporting surface in the window frame and the windscreen itself.
These prior art constructions have a plurality of disadvantages in common. To achieve adequate stability, the prior art knives are produced in one piece and from the same material since the part of the knife which extends from the cutting edge to the tool carrier and which is referred to hereinafter briefly as the shank, has always been made to be narrow and thin so that the cutting edge of the knife can reliably reach the aforementioned inaccessible places.
However, since the knives consist of an especially alloyed material, which is tough, resilient and difficult to machine, manufacture of the connecting portion of the shank was also correspondingly expensive and involved, and generally substantially more material was used for producing the shank than for producing the cutting edge part of the knife. On the other hand, however, the aforementioned material properties which are important for the cutting edge, are of only secondary importance to the shank. It is only necessary to have a certain strength even though the overall dimensions are small, since the knife is subjected to substantial loadings when in use.
Furthermore, in the case of the prior art knives, the axis of rotation is quite remote from the shank part which extends at right-angles to the cutting face. Therefore, also this part performs the oscillatory rotary movements, executing a travel of a few millimetres. In the case of windscreens which are fitted tightly into the vehicle frame, when such a tool is being used, this shank part fits exactly between the edge of the windscreen and the vehicle frame. As a result of the oscillatory rotary movements of this shank part, therefore, it is possible for the vehicle frame and/or the windscreen to become damaged. From this point of view, it would be more sensible for this part of the shank to be disposed close to the axis of rotation or for the axis of rotation even to extend through this part of the shank.